1. Field of the Invention
The present invention relates to a temperature-dependent current sensor circuit and a substantially constant intensity light source and corresponding method using this sensor circuit.
2. Brief Description of the Prior Art
Insertion of an integrated power factor controller circuit such as controller MC33262 from MOTOROLA in an electric power supply system enables easy and efficient control of the power factor and the level of current harmonics.
To obtain a power factor equal to unity, controller MC33262 draws current from the ac source in proportion to the sinusoidal voltage. This concept automatically causes the current waveform to be sinusoidal and in phase with the voltage waveform.
Also, operation of power factor controller MC33262 requires that the output supply voltage be higher than the peak amplitude of the input sinusoidal voltage in order to draw current from the ac source throughout every cycle of the sinusoid. Accordingly, the output supply voltage must have an amplitude higher than the peak amplitude of the sinusoidal voltage of the ac source.
In certain circumstances, an output supply voltage with an amplitude lower than the peak amplitude of the input ac voltage is required. In such cases, power factor controller MC33262 is used as a power-factor-correcting pre-converter; a second power converter is also required to reduce the level of the supply voltage to the desired amplitude.
Necessarily, providing a second power converter involves additional costs and requires more space.
Furthermore, the voltage/current characteristic of a light-emitting diode is sensitive to temperature causing the current through a light-emitting diode to change very rapidly and non-linearly with the voltage across the light-emitting diode.
For example, for a given type of light-omitting diode widely used in the fabrication of traffic signal lights, a constant voltage of 1.8 volts will produce in the light-emitting diode a current of about 7.5 mA at a temperature of -25.degree. C., a current of about 20.5 mA at a temperature of +25.degree. C., and a current of about 30 mA at a temperature of +60.degree. C. The magnitude of the current through the light-emitting diode at a temperature of +60.degree. C. is therefore, for a constant voltage of 1.8 volt, about 1.6 times higher than the magnitude of the current at a temperature of +25.degree. C. Voltage feedback control would therefore be very detrimental to the service life of such a light-emitting diode.
Since voltage feedback control of the supply of a light-emitting diode is not desirable, current feedback control is required to ensure durability of the light-emitting diode.
Also, a fixed LED output current presents the following drawbacks:
at higher temperature the output LED power decreases; and PA1 at lower temperature the output LED power increases.